New Hampshire health officials announced last week that hospitals in three New England states may have accidentally exposed 15 people to prions, the infectious protein that ravages the brain and leaves it full of holes. Evidently, the hospitals involved used surgical tools that had previously been deployed on a patient who officials suspect later died from a particular prion infection called sporadic Creutzfeldt-Jakob disease (CJD).

As disturbing as the revelation was, it pales in comparison with the announcement in 2002, when the University of Pittsburgh Medical Center Presbyterian announced that up to 4,000 patients might have been exposed to the prion. Both incidents show that the hospital transmission of prion diseases remains an ever-present possibility, if thankfully a very unlikely one.

Prions are unusual pathogens distinct from parasites, fungi, bacteria and viruses. They are misfolded proteins that can transform healthy proteins into sickly versions, leading to the death of cells. Particularly abundant in the brain, they took center stage in the late 1980s, during the mad cow outbreak in the U.K. People who ate beef from infected cows ran the risk of contracting a variant of CJD. The panic brought to light the range of prion diseases that can affect humans and animals, including one that develops spontaneously. Called sporadic CJD, this spontaneous form strikes about one in every million people each year for no apparent reason. What’s more, the brain tissue from the unlucky few can infect healthy brains—hence, the worry over surgical transmission.

Ensuring neurosurgical tools are free of prions is difficult chiefly because prions resist standard sterilization procedures. To disinfect metal instruments, hospitals put them in an autoclave and steam-heat them to 121 degrees Celsius for about 15 minutes. That’s far more than what’s needed to wipe out pathogens such as bacteria and viruses, which succumb to mere boiling temperatures in about one minute. Although autoclaving greatly weakens prions, the process may not entirely wipe out these malevolent proteins.

Researchers first learned that the hard way in 1976, when Swiss physician Christopher Bernoulli reported the unsettling news that two young epileptics died of CJD, which generally does not strike until late middle age. Bernoulli had put electrodes into their brains to locate the origins of their seizures. Those same electrodes, it turned out, had been used two years earlier on a 69-year-old CJD patient and had been cleaned with benzene, alcohol and formaldehyde, evidently to no avail.

To confirm that the electrodes did indeed transmit the infection from the CJD patient to the young epilepsy patients, National Institutes of Health researchers plunged two of the electrodes into the brains of chimpanzees. They later got sick and died even though years had passed since the electrodes were first contaminated. The two Swiss cases represent the only confirmed surgical transmissions of prions, but at least nine other cases are suspected.

Another reason why prion-free guarantees are impossible: the disease has a long incubation period—long enough that an infected person would seem quite healthy and arouse no suspicion among hospital staff. Months can pass—and many patients exposed—before surgeons might learn of a CJD case and pull the tools out of use. Currently, researchers have no way of definitively diagnosing sporadic CJD except by autopsy.

Even so, the latest potentially exposed patients—from New Hampshire, Massachusetts and Connecticut—will not likely contract CJD, health officials noted in a statement. "The risk to these individuals is considered extremely low," said José Montero, New Hampshire Department of Health and Human Services director of public health, "but after extensive expert discussion, we could not conclude that there was no risk, so we are taking the step of notifying the patients and providing them with as much information as we can.”

In part, the low risk may be the result of the type of instruments used. In the Swiss patients from the 1970s the transmissions occurred via electrodes implanted deep in their brains for days, giving time for the transmission of infection. In contrast, the New England exposures involved tools that are less intrusive and used only for the duration of the procedures. The tools came with an imaging system that guides surgeons during operations, says Cindy Resman, a spokesperson for Medtronic, which made the instruments used on some of the patients.

Moreover, history might provide some comfort to the exposed individuals: None of the Pittsburgh patients has yet developed CJD in the 11 years since their exposures, although the incubation period for prion diseases can span decades.

Pathologists will examine the brain tissue samples of patient zero to confirm whether he had CJD, says Elizabeth Talbot, an associate professor of infectious disease at Dartmouth College and New Hampshire’s deputy state epidemiologist. “We have more to learn” about CJD, she says, “but thankfully it is a very rare disease.” Currently, there is no plan to test the infectivity of the surgical tools, which remain in quarantine and will likely be destroyed.